1. Field of the Invention
This invention relates to a two filament electric bulb for safe illumination of traffic light signal generators, and more particularly to apparatus for periodically and alternately energizing each of the filaments as a main filament and an auxiliary filament.
2. Description of the Prior Art
It is well recognized in the prior art that under no circumstances should a stop or red light be allowed to fail in traffic light signal generators, since such failure could endanger life and property. It is therefore known to supervise such stop signals with respect to their required operating conditions and to switch off signal lights in the event of failure. Since a traffic signal arrangement would be interrupted, at least partially, in its operation, it has been heretofore a practice to provide reserves in the signal light generators for the stop signals in such a way that either two electric bulbs, or at least electric bulbs with two filaments, are provided.
In the case of two electric bulbs, the stop signal is usually operated with both bulbs simultaneously activated.
Contrary to the above approach, two filament lights are designed in such a way that only one of the two filaments is allowed to be activated at one time. In the case of a failure of the "main" filament, the operation is transferred to the respective auxiliary filament.
The utilization of a traffic light comprising several electric bulbs is not satisfactory, since the signal generator must be larger, and since two illumination fields must be provided for the same signal. In comparison therewith, electric bulbs having two filaments offer the possibility of a space-saving installation, since for the two filaments, which are located close to each other, the same focal point may be assumed. With such a construction, operation takes place according to the principle of operating readiness or redundance. Accordingly, a "main" filament is used regularly and supervised, and in case of failure of the main filament, the "auxiliary" filament is switched on automatically and an alarm signal is activated at the same time.
Recently, halogen electric bulbs have been used as light sources, since halogen bulbs have a substantially higher light yield and a longer life expectancy than normal electric bulbs. In addition, halogen bulbs are free of blackening which otherwise causes a gradual decrease of the light yield. Unfortunately, heretofore such halogen electric bulbs could not be used as two filament lights with one "main" filament and one "auxiliary" filament for traffic light signal generators. Because of the tungsten-halogen cyclical process in these lamps, the relatively cold tungsten parts in the interior of a bulb, also including the auxiliary filaments, were gradually decreased and therefore destroyed.